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Intermediate products characterization and thermal decomposition mechanism of potassium tetraoxalate during in-situ synthesis of potassium carbonate
Journal of Analytical and Applied Pyrolysis xxx (xxxx) xxx–xxx
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Intermediate products characterization and thermal decomposition mechanism of potassium tetraoxalate during in-situ synthesis of potassium carbonate ⁎
Naina Raje , Bhupesh B. Kalekar, Darshana K. Ghonge Analytical Chemistry Division, B.A.R.C., Mumbai – 400 085, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Potassium tetraoxalate dihydrate Thermal analysis In-situ characterization Decomposition mechanism Powder X- ray diffraction (PXRD)
Current studies describe the application of simultaneous TG–DTA–FTIR techniques in the in-situ synthesis of potassium carbonate (K2CO3) from the thermal decomposition of potassium tetraoxalate dihydrate [KH3(C2O4)2.2H2O]. Progress of the decomposition reactions with increasing temperatures was monitored online through TG mass loss, corresponding heat effects and evolved gases. Sequential formation of anhydrous potassium tetraoxalate, potassium hydrogen oxalate [KHC2O4] and potassium oxalate [K2C2O4] was confirmed using thermal, ATR − FTIR and PXRD measurements. The intermediate potassium oxalate undergoes structural transformation and then decomposes to potassium carbonate by the temperature of 600 °C. PXRD measurements confirm the formation of K2CO3 and the results are in agreement with ATR − FTIR and thermal analysis.
1. Introduction Metal oxalates are from an extended class of inorganic compounds that have significant relevance in chemistry and biology both. These compounds are majorly used as precursors for preparing multinary oxides using solid state synthesis. It is required to understand the conditions under which the decomposition reactions proceed and intermediate formations take place during the course of the transformations from an oxalate to the corresponding intermediate carbonates or oxides. In this context, anhydrous alkali-metal oxalates can serve as a source for highly reactive carbonates, which can be utilized as sources of basic oxides [1]. Potassium carbonate is an important raw material in basic inorganic chemical industry, medicine industry and light industry. It has been mainly used in production of optical glass, electrode tube, TV tube, bulb, printing items, dye, ink, photography items, sodium metasilicate, polyester powder, plating, leather, ceramic building materials, crystal, potash soap and drugs [2]. It may be synthesized through the thermal decomposition of potassium tetraoxalate. The thermal degradation of oxalates has been studied extensively by thermo-analytical techniques and the thermal behavior of lithium, potassium, rubidium, and cesium oxalates [3–9] have been studied using differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The DTA/DSC measurements indicate exothermic/endothermic heat effects depending on the environment used in the vicinity of sample holder assembly during the decomposition of oxalate compounds [10,11]. These techniques are
⁎
important in studying the phase transformation of the solid sample in a thermal process but remain unable to provide any information regarding the intermediate/s formed during the thermal process. Moreover, these studies are confined to mostly mono oxalate compounds and the literature seems scarce on the thermal behavior of higher oxalates. It is well known that oxalate compounds have many industrial and analytical applications. Exploration of new applications require an in-depth analysis of different oxalate compounds. Simultaneous TG − DTA − EGA may be applied for the in-situ characterization of the intermediate formed [12]. Potassium tetraoxalate dihydrate is a certified secondary standard reference material for pH measurement; directly traceable to primary reference material NIST/PTB pH(S) = 1.68 (25 °C) (DIN 19266) CertiPUR® [13]. It is widely used for marble grinding and tool polishing. It is a non hygroscopic compound with a well known stoichiometry. To the best of our knowledge, there are no reports on the thermal behavior of potassium tetraoxalate dihydrate. Present studies have been carried out to understand the thermal behavior of potassium tetraoxalate in different environments and to explore its possible newer applications. Details of the studies are presented here. 2. Material and methods 2.1. Material A.R. grade, potassium tetraoxalate dihydrate (KH3 (C2O4)2·2H2O)
Corresponding author. E-mail address: [email protected] (N. Raje).
https://doi.org/10.1016/j.jaap.2017.12.012 Received 11 May 2017; Received in revised form 12 December 2017; Accepted 23 December 2017 0165-2370/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Raje, N., Journal of Analytical and Applied Pyrolysis (2018), https://doi.org/10.1016/j.jaap.2017.12.012
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Fig. 1. TG − DTA plots of potassium tetraoxalate decomposition in air/inert atmosphere Fig. 1 inset: DTA plots of anhydrous potassium oxalate during heating and cooling in air/inert atmosphere..
was obtained from BDH, England. Simultaneous TG–DTA–EGA measurements were carried out on accurately weighed potassium tetraoxalate samples (50–100 mg) in air/inert atmosphere by heating from room temperature to 700 °C at a heating rate of 10 °C min−1. The flow rate of high purity nitrogen was maintained at 100 mL min−1 to transport the volatile products. High purity nitrogen was also used as a protective gas to the thermo − balance at a flow rate of 20 mL min−1.
Tensor 27) was equipped with a KBr beam splitter and a DLaTGS (deuterated L- alanine − doped triglycene sulfate) detector, sealed, and desiccated to minimize purge effects. The samples were placed over the ATR crystal, and maximum pressure was applied using the slip − clutch mechanism. All spectra were collected at 4 cm−1 spectral resolution using sample and background collection time of 1 min each. FTIR data analysis was done using Opus (version 6.0) software from Bruker.
2.2. Instrumental techniques
3. Results and discussion
Experiments have been carried out using Netzsch Thermo-balance (Model No.: STA 409 PC Luxx) coupled to Bruker FTIR system (Model No.: Tensor 27) via a heated Teflon capillary (1 m long, 2 mm i.d.). In this work Pt vs. Pt-10% Rh thermocouples were used as temperature and differential temperature sensors. Re-crystallised alumina sample holders were used as sample and reference holders. TG–DTA data analysis was done using Proteus software from Netzsch. FTIR system used for the identification of IR absorbance in the mid IR region (400–4000 cm−1) is equipped with liquid nitrogen cooled MCT detector and low-volume gas cell (8.7 mL) with a 123 mm path length and KBr windows. The adapter head of thermo-balance, transfer line and sample cell were heated to a constant temperature of 200 °C to avoid condensation of low volatile compounds. The FTIR compartment was continuously purged by high purity nitrogen and molecular sieves/ silica gel were used to minimize the water and carbon dioxide background in the recorded spectra. The resolution of the collected spectra was set to 4 cm−1and co − addition of 32 scans per spectrum with a scan speed of 20 kHz was applied. As a consequence, spectra were recorded with a temporal resolution of about 2.5 s, depending on the integration methods. FTIR data analysis was done using Opus (version 6.0) software from Bruker. Powder X − ray diffraction measurements were carried out on a X − ray diffractometer (Model PW1710, Philips, The Netherlands) using nickel filtered Cu Kα radiation to identify the compounds in 2θ region of 10–70° with a scanning step of 0.02° in 0.8 s. A PIKE MIRacle attenuated total reflection (ATR) accessory equipped with a single reflection diamond ATR crystal was used for the analysis of solid samples. The MIRacle accessory was fitted with a high pressure clamp, providing intimate contact between the sample and the ATR crystal. The MIRacle ATR accessory utilizes a pre − aligned, pinned-in-place crystal plate design enabling easy exchange of the ATR crystal for sampling optimization. The FTIR spectrometer (Bruker
3.1. In-situ synthesis of potassium carbonate In-situ formation of K2CO3 was carried out by the thermal decomposition of accurately weighed (100 mg) potassium tetraoxalate dihydrate in silicon carbide furnace. The compound was heated in a controlled manner with the heating rate of 10 K min−1 in air/inert atmosphere using a flow rate of 100 mL min−1. In-situ decomposition process and completion of this reaction were monitored through the thermogravimetric measurements coupled with simultaneous evolved gas analysis. Toffset temperature and the associated plateau on the TG curve confirmed the completion of the reaction. It can be seen from Fig. 1 that by the temperature of 600 °C, [KH3(C2O4)2·2H2O] decomposes with a mass loss of 73.5% which matches well with the calculated mass loss of 72.8% for the conversion of potassium tetraoxalate dihydrate to potassium carbonate, remaining as 26.5% residue after the completion of the reaction. Table 1 shows the stepwise decomposition process where as Figs. 2, 3 and 4 shows the evolved gas, XRD and ATR − FTIR spectra, in the sequence of the decomposition reaction is progressing. These measurements have been carried out to support the Thermogravimetric analysis and to confirm the formation of potassium carbonate by the temperature of 600 °C. PXRD and ATR − FTIR measurements have been carried out on the residue obtained at 600 °C and Table 1 TG mass loss data of potassium tetraoxalate decomposition in air/inert environment.
2
S.N.
Temperature Region (°C)
Calculated mass loss (%)
Experimental mass loss (%)
1. 2. 3. 4.
80–180 180–240 250–300 525–625
14.2 35.4 17.7 5.5
14.7 35.5 17.8 5.5
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Fig. 2. Evolved gas profile of potassium tetraoxalate decomposition in air/inert atmosphere−. 1. Gram Schmidt curves — A–D. 2. Extracted absorption spectra, corresponding to each Gram Schmidt curve − A1–D1.
3.2. Intermediate characterization and thermal decomposition mechanism
both the techniques confirm the formation of potassium carbonate. The details of the analysis has been discussed in section 3.2 of this work. Besides being simple and efficient, this method of synthesis is having an added advantage over other methods [14]. The synthesized compound is pure as there is no reagent contamination.
Simultaneous TG − DTA curves of potassium tetraoxalate dihydrate [KH3(C2O4)2·2H2O], recorded in nitrogen and air environments are given in Fig. 1. The TG mass loss data is given in Table 1. It can be seen from the TG curve shown in Fig. 1 that the compound is stable upto 80 °C and was completely dehydrated by the temperature of 180 °C with 3
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Fig. 3. XRD patterns of intermediates formed during the decomposition of potassium tetraoxalate −. [A] KH3 (C2O4)2·2H2O; [B] KH3 (C2O4)2; [C] KHC2O4; [D] K2C2O4; [E] K2CO3.
RTXRD pattern Fig. 3A matches with the reported XRD pattern of potassium tetraoxalate dihydrate [16] whereas the recorded XRD pattern for the intermediate formed at 180 °C, is given as Fig. 3B. It can be seen from Fig. 3A and B, that XRD patterns of KH3(C2O4)2·2H2O and KH3(C2O4)2 are entirely different. It may be due to the structural changes during the dehydration process. Similar observation has been reported by Andrzej Kolezynski et al. [17] during their work on various metal oxalate systems. They have reported that at around 200 °C, the C2/c phase of hydrated nickel oxalate transforms into P21/c phase and resulting in the formation of anhydrous nickel oxalate. Similar behavior is reported [18,19] for other metal oxalates also. ATR − FTIR measurements were also carried out at the residue obtained at 180 °C and is given in Fig. 4B along with the absorbance spectra of potassium tetraoxalate dihydrate as Fig. 4A. The comparison between Fig. 4A and B shows that only the vibrational frequencies corresponding to HeOeH stretching/bending are missing in Fig. 4B, reflecting that by the temperature of 180 °C, the dehydration process is complete. These results
a mass loss of 14.7% to form anhydrous potassium tetraoxalate as an intermediate product. It can be inferred from Table 1 that experimental mass loss, in the temperature region of 80–180 °C, is matching well with the calculated mass loss of 14.2% corresponding to the evolution of two water molecules from potassium tetraoxalate dihydrate and forming anhydrous potassium oxalate. Due to this dehydration process, the endothermic heat effect can be seen in the temperature region of 80–180 °C on DTA curves [Fig. 1], recorded in air and inert environments. The evolution of water was confirmed by evolved gas analysis and the simultaneously recorded Gram Schmidt curve along with the extracted FTIR absorption spectra have been given in Fig. 2. The extracted IR absorption spectrum from the Gram Schmidt curve [Fig. 2A] confirms the evolution of water at 148 °C (peak temp.) through the presence of IR absorption bands in the region of 1220–2095 and 3375–4000 cm_1 corresponding to the HeOeH bending/stretching vibrations, respectively [15]. The intermediate/end products were identified using XRD analysis and the spectra have been given Fig. 3. The
Fig. 4. ATR patterns of intermediates formed during the decomposition of potassium tetraoxalate.
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monohydrate [26] except for the frequency region corresponding to HeOeH stretching/bending modes of vibrations, confirms the formation of anhydrous potassium oxalate. Similar results were obtained for the residue obtained at 525 °C, suggests that anhydrous potassium oxalate is formed at 300 °C and stable upto 525 °C. These results are in accordance with the TG results that K2C2O4 is stable upto 525 °C but if we see the DTA curve [Fig. 1] in the temperature region of 350–400 °C, we can see an endothermic heat effect. This observation clearly indicates that K2C2O4 is not thermally inert in this temperature region and undergoing a physical transformation. It can be seen from Fig. 1 that the onset of endothermic effect on DTA curve is at 385 °C. Similar observation has been reported by Robert E. Dinnebier etal.[27] and T. Higashiyama etal. [28] for anhydrous potassium oxalate using DSC and DTA respectively. They followed heating as well as cooling cycle and noted the endothermic heat effects in this temperature region due to the structural phase transformation in K2C2O4. Fig. 1- inset shows the heating and cooling DTA plots for anhydrous potassium oxalate with a large thermal hysteresis. Dinnebier etal. also observed a large hysteresis on DSC curve for this transition. This behavior may be due to kinetically inhibited process with a high activation energy. It has been reported [27] that the endothermic heat effects are due to structural transformation of anhydrous potassium oxalate from low temperature δ phase to high temperature α phase. The oxalate anions are staggered and disordered in the α − phase and disorder increases with the increase in temperature. This disorder at high temperatures of oxalate group provides the significant entropic stabilization to the high temperature phase. It can be seen from Fig. 1 and Table 1 that above the temperature of 525 °C, K2C2O4 decomposes with a mass loss of 5.5% matching well with the calculated mass loss of 5.5% for the formation of potassium carbonate (K2CO3). The evolved gas patterns in the temperature region of 525–630 °C, have been given in Fig. 2C and D in inert and air atmosphere respectively. The simultaneous evolution of CO and CO2 in inert atmosphere [Fig. 2C] may be due to Boudouard reaction where CO/CO2 disproportionation takes place to CO2/CO with variation in temperature. Evolution of only CO2 is seen at 600 °C, in air atmosphere [Fig. 2D] which may be due to the oxidation of evolved CO through the atmospheric oxygen. The corresponding heat effects can be seen on DTA curves, given in Fig. 1. The endothermic heat effect is seen on DTA curve in inert atmosphere though EGA results suggest the disproportionation reaction which is an exothermic process. It should be mentioned that DTA is showing the overall heat effect due to the two competing processes viz. decomposition and disproportionation, occurring simultaneously. The DTA curve, recorded in air atmosphere [Fig. 1] is reflecting an exothermic process in the temperature region of 550–630 °C. It may be attributed to the oxidation of CO, evolved during the decomposition of K2C2O4. The thermal analyses for potassium, rubidium, and cesium oxalate show that the decomposition of these oxalate compounds to the corresponding carbonate phases occurs in a narrow temperature range between 507 and 527 °C with generation of carbon monoxide and of the secondary products carbon dioxide and oxygen [27]. In order to ascertain the stoichiometry of the residue obtained at 650 °C, PXRD analysis has been carried out and is shown in Fig. 3E. The recorded PXRD pattern matches well with the reported PXRD pattern [29] of K2CO3 with JCPDS card no. 0049-1093, confirming the formation of K2CO3 as suggested by thermo analytical techniques. ATR − FTIR analysis has been carried out for the residue obtained at 650 °C and the absorption spectra has been given in Fig. 4E which matches well with the reported ATR − FTIR spectra [30] of K2CO3, resulting in accordance with thermal and PXRD results. On the basis of these studies the thermal decomposition mechanism of potassium tetraoxalate has been proposed and given in Table 2.
are in accordance with TG and EGA measurements. It can be seen from Fig. 1 that anhydrous potassium tetraoxalate, formed at 180 °C, as an intermediate product, decomposes in the temperature region of 180–240 °C with a mass loss of 35.5% along with the evolution of carbon dioxide (CO2) and formic acid (HCOOH) [Fig. 2 B/ B1]. The mass loss data and evolved gas analysis suggest the formation of potassium hydrogen oxalate (KHC2O4), as an intermediate product, by the temperature of 240 °C. The calculated mass loss of 35.4% matches well with the experimental mass loss for the proposed decomposition reaction. Corresponding endothermic heat effects, in the temperature region of 180–240 °C, can be seen on the DTA curves [Fig. 1] in both the atmospheres. The extracted IR absorbance spectra [Fig. 2B1] from the corresponding Gram Schmidt curve [Fig. 2B] in the frequency region of 2215–2430 cm−1 matches well with the standard IR absorbance spectrum of CO2 [20]. The presence of 2354 cm−1 peak is due to O]C]O anti symmetric stretching mode of vibrations. T. Kecskés and co-workers have worked on the FTIR spectroscopy of formic acid in gaseous phase and have reported vibrational frequency bands corresponding to the OeH stretching, CeH stretching, CeO stretching and CeH bending as 3750, 2928, 1671–1658 and 1072–1081 cm−1 respectively [21]. These characteristic modes of vibrations can be seen in the IR absorption spectra [Fig. 2B1] at 215 °C, confirming the evolution of formic acid and supports the TG and DTA analysis. The noticeable observation in the EGA patterns, is the evolution of formic acid at high temperatures like 215 °C [Fig. 2B/B1] and 286 °C [Fig. 2B/B1- inset] instead of H2O/H2 and CO/CO2. It may be due to the fact that formic acid decomposes to CO2 and H2 in the temperature region of 350–550 °C as reported by W. L. Nelson et al. [11]. In order to further support the TG, DTA and EGA results, the residue obtained at 240 °C was analyzed using PXRD and is given in Fig. 3C. It matches with the reported PXRD pattern of KHC2O4 [22] and thus confirms the formation of KHC2O4. ATR − FTIR measurements were carried out for the residue obtained at 240 °C and has been given in Fig. 4C. Comparison between Fig. 4B and C reflects the deviation from the tetraoxalate matrix structure. This shift may be due to the decomposition of anhydrous potassium tetraoxalate by evolution of HCOOH and CO2 as suggested by TG, DTA and EGA measurements. The decomposition process might be leading to the internal rearrangement of atoms and showing the different FTIR pattern for KHC2O4 as compared to that of KH3C2O4. In the temperature region of 250–300 °C, KHC2O4 decomposes [Fig. 1] with a mass loss of 17.8% due to the evolution of carbon dioxide (CO2) and formic acid (HCOOH) [Fig. 2B/B1- inset]. The calculated mass loss of 17.7% matches well with the experimental mass loss for the formation of anhydrous potassium oxalate (K2C2O4). The endothermic heat effects in the corresponding temperature region can be seen on the DTA curves [Fig. 1]. The residue obtained at 300 °C was analyzed using PXRD and is given in Fig. 3D. It matches with the reported PXRD pattern of K2C2O4 [23] and thus confirms the formation of K2C2O4 as an intermediate product due to the decomposition of KHC2O4. These results are in accordance with thermo-analytical results. It can be seen from Fig. 1 that potassium oxalate does not decompose in the temperature region of 300–525 °C. This observation is supported by the ATR − FTIR analysis. The residue of potassium tetraoxalate, obtained at 350 °C was analyzed using ATR − FTIR spectrometry and the absorbance spectrum has been given in Fig. 4. Comparison between Figs. 4A and 4D shows that the vibrational frequencies corresponding to HeOeH stretching/bending modes are missing in Fig. 4D while the characteristic frequencies of oxalate group are seen in both the spectra corresponding to OeCeO antisymmetric and symmetric stretching and OeCeO deformation in the region of 1340–1660 and 760–770 cm−1 respectively [24]. It has been reported by B.F. Pedersen [25] that in the absence of hydrogen bonding, anhydrous oxalate compounds like Na2C2O4, show only the bands corresponding to pure oxalate ion and do not show any band above 1660 cm−1. The ATR − FTIR spectrum in 4D matches well with the reported FTIR spectra of potassium oxalate
4. Conclusions Analysis of evolved gases using FTIR played an important role in 5
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Table 2 Proposed Decomposition mechanism of potassium tetraoxalate. S.N.
Reactant
A. 1. 2. 3. 4. 5. B. 1. 2.
Inert environment − 2KH3 (C2O4)2·2H2O 2KH3 (C2O4)2 2KHC2O4 K2C2O4 (s) K2C2O4 (s) Air environment − K2C2O4 (s) CO (g) + ½ O2 (g)
Temperature (°C)
Calculated Mass Loss (%)
Experimental Mass loss (%)
Decomposition Products
80–180 °C 180–240 °C 250–300 °C 350–400 °C 490–600 °C
14.2 35.4 17.8 Phase transition 5.5
14.7 35.5 17.7
2KH3 (C2O4)2 + 4H2O 2KHC2O4 + 2CO2 + 2HCOOH K2C2O4 + CO2 + HCOOH K2C2O4 (s) K2CO3 (s) + CO (g)
490–600 °C 490–600 °C
5.5 5.5 Oxidation of evolved CO during 490–600 °C in air environment
5.5
understanding the decomposition reaction mechanism of potassium tetraoxalate. Present studies confirm the sequential formation of anhydrous potassium tetraoxalate, potassium hydrogen oxalate and potassium oxalate during the decomposition of potassium tetraoxalate. Potassium oxalate formed during the decomposition, undergoes phase transformation and then decomposes to potassium carbonate by the temperature of 600 °C. This method may be used for the in-situ formation of anhydrous potassium oxalate or potassium carbonate with high purity as the method does not involve any reagent contamination. Our studies suggest that potassium tetraoxalate may be used as a suitable calibrant for temperature or mass change calibrations in simultaneous TG − DTA thermal analyzer systems because of its thermal and physical properties like known stoichiometry and non hygroscopic nature.
K2CO3 (s) + CO (g) CO2 (g)
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Acknowledgements Authors are thankful to Dr. P.D. Naik, Head, ACD and Associate Director, Chemistry Group, BARC for his constant support and encouragement. We would like to acknowledge Dr. A. K. Tyagi, Head, Structural Chemistry Section, ChD, BARC for providing PXRD facility. References [1] M.J. Munson, R.E. Riman, Observed phase transformations of oxalate-derived lead monoxide powder, J. Therm. Anal. 37 (11–12) (1991) 2555–2566. [2] Uses and application of potassium carbonate https://b2bbusinessnews.wordpress. com/2012/06/18/uses-and-applications-of-potassium-carbonate/. [3] D. Dollimore, The thermal decomposition of oxalates: a review, Thermochim. Acta 117 (1987) 331–363. [4] D. Dollimore, The production of metals and alloys by the decomposition of oxysalts, Thermochim. Acta 177 (1991) 59–75. [5] A.K. Galwey, M.E. Brown, Thermal Decomposition of Inorganic Solids, Ch. 16, Elsevier, Amsterdam, 1999. [6] D. Dollimore, D. Tinsley, The thermal decomposition of oxalates. Part XII. The thermal decomposition of lithium oxalate, J. Chem. Soc. A (1971) 3043–3047. [7] Y. Masuda, R. Ito, T. Matsuda, Y. Ito, The thermal phase transition of anhydrous potassium oxalate, Thermochim. Acta 131 (1988) 291–296. [8] I.A. Kahwa, A.M. Mulokozi, The thermal decomposition temperatures of ionic metal oxalates, J. Therm. Anal. 22 (1981) 61–65. [9] R. Ito, Y. Masuda, Y. Ito, Thermal analysis of rubidium and cesium oxalate monohydrates, Thermochim. Acta 127 (1988) 159–170. [10] V. Balek, N.A. Karabascheva, K. GyÖryová, Literature survey on thermal analysis reference materials, J. Therm. Anal. 40 (1993) 1459–1463. [11] W.L. Nelson, C.J. Engelder, The thermal decomposition of formic acid, J. Phys.
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Intermediate products characterization and thermal decomposition mechanism of potassium tetraoxalate during in-situ synthesis of potassium carbonate ⁎
Naina Raje , Bhupesh B. Kalekar, Darshana K. Ghonge Analytical Chemistry Division, B.A.R.C., Mumbai – 400 085, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Potassium tetraoxalate dihydrate Thermal analysis In-situ characterization Decomposition mechanism Powder X- ray diffraction (PXRD)
Current studies describe the application of simultaneous TG–DTA–FTIR techniques in the in-situ synthesis of potassium carbonate (K2CO3) from the thermal decomposition of potassium tetraoxalate dihydrate [KH3(C2O4)2.2H2O]. Progress of the decomposition reactions with increasing temperatures was monitored online through TG mass loss, corresponding heat effects and evolved gases. Sequential formation of anhydrous potassium tetraoxalate, potassium hydrogen oxalate [KHC2O4] and potassium oxalate [K2C2O4] was confirmed using thermal, ATR − FTIR and PXRD measurements. The intermediate potassium oxalate undergoes structural transformation and then decomposes to potassium carbonate by the temperature of 600 °C. PXRD measurements confirm the formation of K2CO3 and the results are in agreement with ATR − FTIR and thermal analysis.
1. Introduction Metal oxalates are from an extended class of inorganic compounds that have significant relevance in chemistry and biology both. These compounds are majorly used as precursors for preparing multinary oxides using solid state synthesis. It is required to understand the conditions under which the decomposition reactions proceed and intermediate formations take place during the course of the transformations from an oxalate to the corresponding intermediate carbonates or oxides. In this context, anhydrous alkali-metal oxalates can serve as a source for highly reactive carbonates, which can be utilized as sources of basic oxides [1]. Potassium carbonate is an important raw material in basic inorganic chemical industry, medicine industry and light industry. It has been mainly used in production of optical glass, electrode tube, TV tube, bulb, printing items, dye, ink, photography items, sodium metasilicate, polyester powder, plating, leather, ceramic building materials, crystal, potash soap and drugs [2]. It may be synthesized through the thermal decomposition of potassium tetraoxalate. The thermal degradation of oxalates has been studied extensively by thermo-analytical techniques and the thermal behavior of lithium, potassium, rubidium, and cesium oxalates [3–9] have been studied using differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The DTA/DSC measurements indicate exothermic/endothermic heat effects depending on the environment used in the vicinity of sample holder assembly during the decomposition of oxalate compounds [10,11]. These techniques are
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important in studying the phase transformation of the solid sample in a thermal process but remain unable to provide any information regarding the intermediate/s formed during the thermal process. Moreover, these studies are confined to mostly mono oxalate compounds and the literature seems scarce on the thermal behavior of higher oxalates. It is well known that oxalate compounds have many industrial and analytical applications. Exploration of new applications require an in-depth analysis of different oxalate compounds. Simultaneous TG − DTA − EGA may be applied for the in-situ characterization of the intermediate formed [12]. Potassium tetraoxalate dihydrate is a certified secondary standard reference material for pH measurement; directly traceable to primary reference material NIST/PTB pH(S) = 1.68 (25 °C) (DIN 19266) CertiPUR® [13]. It is widely used for marble grinding and tool polishing. It is a non hygroscopic compound with a well known stoichiometry. To the best of our knowledge, there are no reports on the thermal behavior of potassium tetraoxalate dihydrate. Present studies have been carried out to understand the thermal behavior of potassium tetraoxalate in different environments and to explore its possible newer applications. Details of the studies are presented here. 2. Material and methods 2.1. Material A.R. grade, potassium tetraoxalate dihydrate (KH3 (C2O4)2·2H2O)
Corresponding author. E-mail address: [email protected] (N. Raje).
https://doi.org/10.1016/j.jaap.2017.12.012 Received 11 May 2017; Received in revised form 12 December 2017; Accepted 23 December 2017 0165-2370/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Raje, N., Journal of Analytical and Applied Pyrolysis (2018), https://doi.org/10.1016/j.jaap.2017.12.012
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Fig. 1. TG − DTA plots of potassium tetraoxalate decomposition in air/inert atmosphere Fig. 1 inset: DTA plots of anhydrous potassium oxalate during heating and cooling in air/inert atmosphere..
was obtained from BDH, England. Simultaneous TG–DTA–EGA measurements were carried out on accurately weighed potassium tetraoxalate samples (50–100 mg) in air/inert atmosphere by heating from room temperature to 700 °C at a heating rate of 10 °C min−1. The flow rate of high purity nitrogen was maintained at 100 mL min−1 to transport the volatile products. High purity nitrogen was also used as a protective gas to the thermo − balance at a flow rate of 20 mL min−1.
Tensor 27) was equipped with a KBr beam splitter and a DLaTGS (deuterated L- alanine − doped triglycene sulfate) detector, sealed, and desiccated to minimize purge effects. The samples were placed over the ATR crystal, and maximum pressure was applied using the slip − clutch mechanism. All spectra were collected at 4 cm−1 spectral resolution using sample and background collection time of 1 min each. FTIR data analysis was done using Opus (version 6.0) software from Bruker.
2.2. Instrumental techniques
3. Results and discussion
Experiments have been carried out using Netzsch Thermo-balance (Model No.: STA 409 PC Luxx) coupled to Bruker FTIR system (Model No.: Tensor 27) via a heated Teflon capillary (1 m long, 2 mm i.d.). In this work Pt vs. Pt-10% Rh thermocouples were used as temperature and differential temperature sensors. Re-crystallised alumina sample holders were used as sample and reference holders. TG–DTA data analysis was done using Proteus software from Netzsch. FTIR system used for the identification of IR absorbance in the mid IR region (400–4000 cm−1) is equipped with liquid nitrogen cooled MCT detector and low-volume gas cell (8.7 mL) with a 123 mm path length and KBr windows. The adapter head of thermo-balance, transfer line and sample cell were heated to a constant temperature of 200 °C to avoid condensation of low volatile compounds. The FTIR compartment was continuously purged by high purity nitrogen and molecular sieves/ silica gel were used to minimize the water and carbon dioxide background in the recorded spectra. The resolution of the collected spectra was set to 4 cm−1and co − addition of 32 scans per spectrum with a scan speed of 20 kHz was applied. As a consequence, spectra were recorded with a temporal resolution of about 2.5 s, depending on the integration methods. FTIR data analysis was done using Opus (version 6.0) software from Bruker. Powder X − ray diffraction measurements were carried out on a X − ray diffractometer (Model PW1710, Philips, The Netherlands) using nickel filtered Cu Kα radiation to identify the compounds in 2θ region of 10–70° with a scanning step of 0.02° in 0.8 s. A PIKE MIRacle attenuated total reflection (ATR) accessory equipped with a single reflection diamond ATR crystal was used for the analysis of solid samples. The MIRacle accessory was fitted with a high pressure clamp, providing intimate contact between the sample and the ATR crystal. The MIRacle ATR accessory utilizes a pre − aligned, pinned-in-place crystal plate design enabling easy exchange of the ATR crystal for sampling optimization. The FTIR spectrometer (Bruker
3.1. In-situ synthesis of potassium carbonate In-situ formation of K2CO3 was carried out by the thermal decomposition of accurately weighed (100 mg) potassium tetraoxalate dihydrate in silicon carbide furnace. The compound was heated in a controlled manner with the heating rate of 10 K min−1 in air/inert atmosphere using a flow rate of 100 mL min−1. In-situ decomposition process and completion of this reaction were monitored through the thermogravimetric measurements coupled with simultaneous evolved gas analysis. Toffset temperature and the associated plateau on the TG curve confirmed the completion of the reaction. It can be seen from Fig. 1 that by the temperature of 600 °C, [KH3(C2O4)2·2H2O] decomposes with a mass loss of 73.5% which matches well with the calculated mass loss of 72.8% for the conversion of potassium tetraoxalate dihydrate to potassium carbonate, remaining as 26.5% residue after the completion of the reaction. Table 1 shows the stepwise decomposition process where as Figs. 2, 3 and 4 shows the evolved gas, XRD and ATR − FTIR spectra, in the sequence of the decomposition reaction is progressing. These measurements have been carried out to support the Thermogravimetric analysis and to confirm the formation of potassium carbonate by the temperature of 600 °C. PXRD and ATR − FTIR measurements have been carried out on the residue obtained at 600 °C and Table 1 TG mass loss data of potassium tetraoxalate decomposition in air/inert environment.
2
S.N.
Temperature Region (°C)
Calculated mass loss (%)
Experimental mass loss (%)
1. 2. 3. 4.
80–180 180–240 250–300 525–625
14.2 35.4 17.7 5.5
14.7 35.5 17.8 5.5
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Fig. 2. Evolved gas profile of potassium tetraoxalate decomposition in air/inert atmosphere−. 1. Gram Schmidt curves — A–D. 2. Extracted absorption spectra, corresponding to each Gram Schmidt curve − A1–D1.
3.2. Intermediate characterization and thermal decomposition mechanism
both the techniques confirm the formation of potassium carbonate. The details of the analysis has been discussed in section 3.2 of this work. Besides being simple and efficient, this method of synthesis is having an added advantage over other methods [14]. The synthesized compound is pure as there is no reagent contamination.
Simultaneous TG − DTA curves of potassium tetraoxalate dihydrate [KH3(C2O4)2·2H2O], recorded in nitrogen and air environments are given in Fig. 1. The TG mass loss data is given in Table 1. It can be seen from the TG curve shown in Fig. 1 that the compound is stable upto 80 °C and was completely dehydrated by the temperature of 180 °C with 3
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Fig. 3. XRD patterns of intermediates formed during the decomposition of potassium tetraoxalate −. [A] KH3 (C2O4)2·2H2O; [B] KH3 (C2O4)2; [C] KHC2O4; [D] K2C2O4; [E] K2CO3.
RTXRD pattern Fig. 3A matches with the reported XRD pattern of potassium tetraoxalate dihydrate [16] whereas the recorded XRD pattern for the intermediate formed at 180 °C, is given as Fig. 3B. It can be seen from Fig. 3A and B, that XRD patterns of KH3(C2O4)2·2H2O and KH3(C2O4)2 are entirely different. It may be due to the structural changes during the dehydration process. Similar observation has been reported by Andrzej Kolezynski et al. [17] during their work on various metal oxalate systems. They have reported that at around 200 °C, the C2/c phase of hydrated nickel oxalate transforms into P21/c phase and resulting in the formation of anhydrous nickel oxalate. Similar behavior is reported [18,19] for other metal oxalates also. ATR − FTIR measurements were also carried out at the residue obtained at 180 °C and is given in Fig. 4B along with the absorbance spectra of potassium tetraoxalate dihydrate as Fig. 4A. The comparison between Fig. 4A and B shows that only the vibrational frequencies corresponding to HeOeH stretching/bending are missing in Fig. 4B, reflecting that by the temperature of 180 °C, the dehydration process is complete. These results
a mass loss of 14.7% to form anhydrous potassium tetraoxalate as an intermediate product. It can be inferred from Table 1 that experimental mass loss, in the temperature region of 80–180 °C, is matching well with the calculated mass loss of 14.2% corresponding to the evolution of two water molecules from potassium tetraoxalate dihydrate and forming anhydrous potassium oxalate. Due to this dehydration process, the endothermic heat effect can be seen in the temperature region of 80–180 °C on DTA curves [Fig. 1], recorded in air and inert environments. The evolution of water was confirmed by evolved gas analysis and the simultaneously recorded Gram Schmidt curve along with the extracted FTIR absorption spectra have been given in Fig. 2. The extracted IR absorption spectrum from the Gram Schmidt curve [Fig. 2A] confirms the evolution of water at 148 °C (peak temp.) through the presence of IR absorption bands in the region of 1220–2095 and 3375–4000 cm_1 corresponding to the HeOeH bending/stretching vibrations, respectively [15]. The intermediate/end products were identified using XRD analysis and the spectra have been given Fig. 3. The
Fig. 4. ATR patterns of intermediates formed during the decomposition of potassium tetraoxalate.
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monohydrate [26] except for the frequency region corresponding to HeOeH stretching/bending modes of vibrations, confirms the formation of anhydrous potassium oxalate. Similar results were obtained for the residue obtained at 525 °C, suggests that anhydrous potassium oxalate is formed at 300 °C and stable upto 525 °C. These results are in accordance with the TG results that K2C2O4 is stable upto 525 °C but if we see the DTA curve [Fig. 1] in the temperature region of 350–400 °C, we can see an endothermic heat effect. This observation clearly indicates that K2C2O4 is not thermally inert in this temperature region and undergoing a physical transformation. It can be seen from Fig. 1 that the onset of endothermic effect on DTA curve is at 385 °C. Similar observation has been reported by Robert E. Dinnebier etal.[27] and T. Higashiyama etal. [28] for anhydrous potassium oxalate using DSC and DTA respectively. They followed heating as well as cooling cycle and noted the endothermic heat effects in this temperature region due to the structural phase transformation in K2C2O4. Fig. 1- inset shows the heating and cooling DTA plots for anhydrous potassium oxalate with a large thermal hysteresis. Dinnebier etal. also observed a large hysteresis on DSC curve for this transition. This behavior may be due to kinetically inhibited process with a high activation energy. It has been reported [27] that the endothermic heat effects are due to structural transformation of anhydrous potassium oxalate from low temperature δ phase to high temperature α phase. The oxalate anions are staggered and disordered in the α − phase and disorder increases with the increase in temperature. This disorder at high temperatures of oxalate group provides the significant entropic stabilization to the high temperature phase. It can be seen from Fig. 1 and Table 1 that above the temperature of 525 °C, K2C2O4 decomposes with a mass loss of 5.5% matching well with the calculated mass loss of 5.5% for the formation of potassium carbonate (K2CO3). The evolved gas patterns in the temperature region of 525–630 °C, have been given in Fig. 2C and D in inert and air atmosphere respectively. The simultaneous evolution of CO and CO2 in inert atmosphere [Fig. 2C] may be due to Boudouard reaction where CO/CO2 disproportionation takes place to CO2/CO with variation in temperature. Evolution of only CO2 is seen at 600 °C, in air atmosphere [Fig. 2D] which may be due to the oxidation of evolved CO through the atmospheric oxygen. The corresponding heat effects can be seen on DTA curves, given in Fig. 1. The endothermic heat effect is seen on DTA curve in inert atmosphere though EGA results suggest the disproportionation reaction which is an exothermic process. It should be mentioned that DTA is showing the overall heat effect due to the two competing processes viz. decomposition and disproportionation, occurring simultaneously. The DTA curve, recorded in air atmosphere [Fig. 1] is reflecting an exothermic process in the temperature region of 550–630 °C. It may be attributed to the oxidation of CO, evolved during the decomposition of K2C2O4. The thermal analyses for potassium, rubidium, and cesium oxalate show that the decomposition of these oxalate compounds to the corresponding carbonate phases occurs in a narrow temperature range between 507 and 527 °C with generation of carbon monoxide and of the secondary products carbon dioxide and oxygen [27]. In order to ascertain the stoichiometry of the residue obtained at 650 °C, PXRD analysis has been carried out and is shown in Fig. 3E. The recorded PXRD pattern matches well with the reported PXRD pattern [29] of K2CO3 with JCPDS card no. 0049-1093, confirming the formation of K2CO3 as suggested by thermo analytical techniques. ATR − FTIR analysis has been carried out for the residue obtained at 650 °C and the absorption spectra has been given in Fig. 4E which matches well with the reported ATR − FTIR spectra [30] of K2CO3, resulting in accordance with thermal and PXRD results. On the basis of these studies the thermal decomposition mechanism of potassium tetraoxalate has been proposed and given in Table 2.
are in accordance with TG and EGA measurements. It can be seen from Fig. 1 that anhydrous potassium tetraoxalate, formed at 180 °C, as an intermediate product, decomposes in the temperature region of 180–240 °C with a mass loss of 35.5% along with the evolution of carbon dioxide (CO2) and formic acid (HCOOH) [Fig. 2 B/ B1]. The mass loss data and evolved gas analysis suggest the formation of potassium hydrogen oxalate (KHC2O4), as an intermediate product, by the temperature of 240 °C. The calculated mass loss of 35.4% matches well with the experimental mass loss for the proposed decomposition reaction. Corresponding endothermic heat effects, in the temperature region of 180–240 °C, can be seen on the DTA curves [Fig. 1] in both the atmospheres. The extracted IR absorbance spectra [Fig. 2B1] from the corresponding Gram Schmidt curve [Fig. 2B] in the frequency region of 2215–2430 cm−1 matches well with the standard IR absorbance spectrum of CO2 [20]. The presence of 2354 cm−1 peak is due to O]C]O anti symmetric stretching mode of vibrations. T. Kecskés and co-workers have worked on the FTIR spectroscopy of formic acid in gaseous phase and have reported vibrational frequency bands corresponding to the OeH stretching, CeH stretching, CeO stretching and CeH bending as 3750, 2928, 1671–1658 and 1072–1081 cm−1 respectively [21]. These characteristic modes of vibrations can be seen in the IR absorption spectra [Fig. 2B1] at 215 °C, confirming the evolution of formic acid and supports the TG and DTA analysis. The noticeable observation in the EGA patterns, is the evolution of formic acid at high temperatures like 215 °C [Fig. 2B/B1] and 286 °C [Fig. 2B/B1- inset] instead of H2O/H2 and CO/CO2. It may be due to the fact that formic acid decomposes to CO2 and H2 in the temperature region of 350–550 °C as reported by W. L. Nelson et al. [11]. In order to further support the TG, DTA and EGA results, the residue obtained at 240 °C was analyzed using PXRD and is given in Fig. 3C. It matches with the reported PXRD pattern of KHC2O4 [22] and thus confirms the formation of KHC2O4. ATR − FTIR measurements were carried out for the residue obtained at 240 °C and has been given in Fig. 4C. Comparison between Fig. 4B and C reflects the deviation from the tetraoxalate matrix structure. This shift may be due to the decomposition of anhydrous potassium tetraoxalate by evolution of HCOOH and CO2 as suggested by TG, DTA and EGA measurements. The decomposition process might be leading to the internal rearrangement of atoms and showing the different FTIR pattern for KHC2O4 as compared to that of KH3C2O4. In the temperature region of 250–300 °C, KHC2O4 decomposes [Fig. 1] with a mass loss of 17.8% due to the evolution of carbon dioxide (CO2) and formic acid (HCOOH) [Fig. 2B/B1- inset]. The calculated mass loss of 17.7% matches well with the experimental mass loss for the formation of anhydrous potassium oxalate (K2C2O4). The endothermic heat effects in the corresponding temperature region can be seen on the DTA curves [Fig. 1]. The residue obtained at 300 °C was analyzed using PXRD and is given in Fig. 3D. It matches with the reported PXRD pattern of K2C2O4 [23] and thus confirms the formation of K2C2O4 as an intermediate product due to the decomposition of KHC2O4. These results are in accordance with thermo-analytical results. It can be seen from Fig. 1 that potassium oxalate does not decompose in the temperature region of 300–525 °C. This observation is supported by the ATR − FTIR analysis. The residue of potassium tetraoxalate, obtained at 350 °C was analyzed using ATR − FTIR spectrometry and the absorbance spectrum has been given in Fig. 4. Comparison between Figs. 4A and 4D shows that the vibrational frequencies corresponding to HeOeH stretching/bending modes are missing in Fig. 4D while the characteristic frequencies of oxalate group are seen in both the spectra corresponding to OeCeO antisymmetric and symmetric stretching and OeCeO deformation in the region of 1340–1660 and 760–770 cm−1 respectively [24]. It has been reported by B.F. Pedersen [25] that in the absence of hydrogen bonding, anhydrous oxalate compounds like Na2C2O4, show only the bands corresponding to pure oxalate ion and do not show any band above 1660 cm−1. The ATR − FTIR spectrum in 4D matches well with the reported FTIR spectra of potassium oxalate
4. Conclusions Analysis of evolved gases using FTIR played an important role in 5
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Table 2 Proposed Decomposition mechanism of potassium tetraoxalate. S.N.
Reactant
A. 1. 2. 3. 4. 5. B. 1. 2.
Inert environment − 2KH3 (C2O4)2·2H2O 2KH3 (C2O4)2 2KHC2O4 K2C2O4 (s) K2C2O4 (s) Air environment − K2C2O4 (s) CO (g) + ½ O2 (g)
Temperature (°C)
Calculated Mass Loss (%)
Experimental Mass loss (%)
Decomposition Products
80–180 °C 180–240 °C 250–300 °C 350–400 °C 490–600 °C
14.2 35.4 17.8 Phase transition 5.5
14.7 35.5 17.7
2KH3 (C2O4)2 + 4H2O 2KHC2O4 + 2CO2 + 2HCOOH K2C2O4 + CO2 + HCOOH K2C2O4 (s) K2CO3 (s) + CO (g)
490–600 °C 490–600 °C
5.5 5.5 Oxidation of evolved CO during 490–600 °C in air environment
5.5
understanding the decomposition reaction mechanism of potassium tetraoxalate. Present studies confirm the sequential formation of anhydrous potassium tetraoxalate, potassium hydrogen oxalate and potassium oxalate during the decomposition of potassium tetraoxalate. Potassium oxalate formed during the decomposition, undergoes phase transformation and then decomposes to potassium carbonate by the temperature of 600 °C. This method may be used for the in-situ formation of anhydrous potassium oxalate or potassium carbonate with high purity as the method does not involve any reagent contamination. Our studies suggest that potassium tetraoxalate may be used as a suitable calibrant for temperature or mass change calibrations in simultaneous TG − DTA thermal analyzer systems because of its thermal and physical properties like known stoichiometry and non hygroscopic nature.
K2CO3 (s) + CO (g) CO2 (g)
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